(a) Field of the Invention
The present invention relates to an austenitic Co-containing stainless steel alloy having a very high resistance to high intensity cavitation erosion.
(b) Brief Description of the Prior Art
U.S. Pat. No. 4,588,440 issued on May 13, 1986 in the name of the present inventor discloses a soft austenitic Co-containing stainless steel alloy having a very high resistance to high intensity cavitation making it particularly useful for the manufacture and/or repair of hydraulic machine components.
The soft stainless steel alloy disclosed and claimed in the U.S. Pat. No. 4,588,440 is characterized in that, on the one hand, it consists essentially of
from 8 to 30% by weight of Co; PA0 from 13 to 30% by weight of Cr; PA0 from 0.03 to 0.3% by weight of C; PA0 up to 0.3% by weight of N; PA0 up to 3% by weight of Si; PA0 up to 1% by weight of Ni; PA0 up to 2% by weight of Mo; and PA0 up to 9% by weight of Mn; PA0 from 13 to 30% by weight of Cr; PA0 from 0.03 to 2.0% by weight of C; PA0 up to now 0.3% by weight of N; PA0 up to 5% by weight of Si; PA0 up to 1.0% by weight of Ni; PA0 up to 2.0% by weight of Mo; and PA0 up to 16% by weight of Mn, PA0 the balance being substantially Fe; PA0 the amount of C is higher than 0.3%; PA0 the amount of Si is higher than 3.0%; PA0 the amount of Mn is higher than 9.0%; and PA0 from 10 to 12% by weight of Co; PA0 from 16 to 18% by weight of Cr; PA0 from 0.4 to 0.5% by weight of C; PA0 from 2.5 to 3.5% by weight of Si; and PA0 from 4.5 to 5.5% by weight of Mn; PA0 the balance being substantially Fe, with traces of impurities such as N or Mo of course, the respective amount of each of the above elements is selected and balanced as explained hereinabove.
the balance being substantially Fe, and, on the other hand, the amount of the above mentioned elements that are respectively known as ferrite formers (Cr, Mo, Si) and as austenite formers (C, N, Co, Ni, Mn) and, amongst said austenite and ferrite formers the amount of each of the elements that are respectively known to increase and lower the stacking fault energy being respectively selected and balanced so that at least 60% by weight of the alloy is, at ambient temperature, in a metastable, face centered cubic (F.C.C.) phase having a stacking fault energy low enough to make it capable of being transformed under cavitation exposure to a fine deformation twinning, hexagonal close pack (H.C.P.) .epsilon.-phase and/or .alpha.-martensitic phase.
As indicated in column 3 of U.S. Pat. No. 4,588,440, the above mentioned composition and the very specific structure of this stainless steel alloy have been "selected" by the inventor after numerous studies and tests carried out following the discovery that soft stainless steel alloys containing as low as 8% by weight of Co have an outstanding cavitation resistance similar to the one obtained with the alloys containing up to 65% by weight of Co, provided that at least 60% of this low Co, stainless steel alloys be at ambient temperature in a metastable, F.C.C. .alpha.-phase having a stacking fault energy low enough to make it capable of being transformed under cavitation exposure to a H.C.P. .gamma.-phase and/or to a .epsilon.-martensitic phase showing a fine deformation twinning.
More particularly, it was then surprisingly discovered that soft Fe-Cr-Co-C alloys showing a fine, cavitation-induced twinning, which twinning is specific to crystals having low stacking fault energy (SFE), resist cavitation in a very efficient manner through the following mechanism:
high strain hardening and strain accomodation, delaying fatigue crack initiation;
extension of planar twinning to the whole surface keeping it quite smooth and flat through an incubation period; and
continuous absorption of the incident cavitation energy by the production of high dislocation density and very fine eroded particles, thus leading to very low erosion rates.